Route Redistribution - Get different routing protocols to pass routing information to each other
	There will come a time when you will have to connect networks running different routing protocols. In a perfect world there would only be protocol that everyone would use for all routing tasks. Route redistribution allows different routing protocols to exchange routing information. There are several reasons why you may need to run multiple routing protocols on a network. You may need to migrate from one routing protocol to another, or you may want to use a new protocol, but need to maintain the current one. Corporate mergers or acquisitions can also introduce different routing protocols into a single network. Using routers from different vendors is another reason that different protocols may be used. If a single routing protocol cannot be used, route redistribution is the only solution. Each routing protocol on a network is separated into an autonomous system (AS). All routers in the same autonomous system (running the same routing protocol) have complete knowledge of the entire AS. A router that connects two (or more) autonomous systems is known as a border router. A border router advertises routing information from one AS to the other AS(s). Before considering redistribution, there are some important factors to consider. It is only possible to redistribute routing information for like routed protocols. That is, you cannot redistribute IPX into IP (you would need a protocol gateway for that). Also, if you have multiple border routers, careful planning is necessary to avoid routing loops (information learned from one AS being sent back to the same AS). Different routing protocols converge at different rates may cause routing update lags in the different ASs. Different routing protocols have different, and often incompatible algorithms and metrics. RIP expresses routes in terms of hop count, while EIGRP's metric is based on several factors (bandwidth, delay, load, reliability and MTU). With route redistribution, the metric of one protocol has to be translated into a metric understandable by the other protocol. Setting a default, or seed metric does this. The metric configured must be understandable to the receiving protocol. If a router learns a route to a destination from more than one protocol, administrative distance determines which route will be entered into the routing table. Administrative distance is a measure of believability. Actual values for Administrative distance can be found here. No matter what routing protocols are being redistributed, there is a basic set of steps that should be followed to configure redistribution. 1) Locate the boundary router(s). This is the router where redistribution will be configured. 2) Choose a protocol to be the core (backbone protocol). A general rule of thumb is to pick OSPF or EIGRP. 3) Determine the edge protocol. If you are migrating, this is the protocol that will be phased out. 4) Configure the router to redistribute from the edge protocol to the backbone protocol. 5) Configure the router to redistribute from the backbone protocol to the edge protocol. 6) Check redistribution by using the show route command. Step 5 may be omitted if a static route can be set up for the edge autonomous system. Using a static route in this way can make configuring redistribution simpler and less prone to errors. Cisco documentation - Redistributing Routing Protocols